Chemical looping combustion of liquid feeds consists in contacting a liquid hydrocarbon feed with a metal oxide at high temperature. The metal oxide then yields part of the oxygen it contains, which takes part in the combustion of the hydrocarbons. After this combustion, the fumes contain predominantly carbon oxides, water and possibly hydrogen. In fact, it is not necessary to contact the air with the hydrocarbons, and the fumes are then predominantly made up of combustion gases and possibly of a dilution gas used for transport and fluidization of the particles (water vapour for example). It is thus possible to produce predominantly nitrogen-free fumes with high CO2 contents (>90 vol. %) allowing to consider CO2 capture, then storage. The metal oxide that has taken part in the combustion is then transported to another reaction enclosure where it is contacted with air in order to be re-oxidized. If the particles from the combustion zone are free of fuel, the gases coming from this reaction zone are predominantly free of CO2—which is then present only as traces, for example at concentrations below 1-2 vol. %—and they essentially consist of oxygen-depleted air, as a result of the oxidation of the metal particles.
Implementing a chemical looping combustion method requires large amounts of metal oxides in contact with the fuel. These metal oxides are generally contained either in ore particles or in particles resulting from industrial treatments (residues from the iron and steel industry or from the mining industry, used catalysts from the chemical industry or refining). It is also possible to use synthetic materials such as, for example, alumina or silica-alumina supports on which metals that can be oxidized (nickel oxide for example) have been deposited. Depending on the metal oxides used, the amount of oxygen theoretically available varies considerably and it can reach high values close to 30%. However, according to the materials, the maximum capacity of oxygen really available does generally not exceed more than 20% of the oxygen present. The capacity of these materials to yield oxygen does therefore globally not exceed more than some percents by weight of the particles and it varies considerably from one oxide to another, generally ranging from 0.1 to 10%, often between 0.3 and 1 wt. %. Implementation with a fluidized bed is therefore particularly advantageous for conducting the combustion. In fact, the finely divided oxide particles circulate more readily in the combustion and oxidation reaction enclosures, and between these enclosures, if the properties of a fluid are conferred on the particles (fluidization).
Chemical looping combustion allows to produce energy, in form of vapour or electricity for example. The feed combustion heat is similar to that encountered in conventional combustion. It corresponds to the sum of the combustion and oxidation heats in the chemical loop. The distribution among the combustion and oxidation heats greatly depends on the metal oxides used for conducting the chemical looping combustion. In some cases, the exothermicity is distributed among the oxidation and the reduction of the metal. In other cases, the oxidation is highly exothermic and the reduction is endothermic. In any case, the sum of the oxidation and reduction heats is equal to the combustion heat of the fuel. The heat is extracted by exchangers arranged inside, on the wall or added to the combustion and/or oxidation enclosures, on the fume lines or on the metal oxide transfer lines.
The combustion of solid feeds and of gaseous feeds is well described in the literature. The principle of the combustion of liquid feeds is known (patent application FR-08/02,450). However, in relation to the gaseous and solid feeds, chemical looping combustion of liquid feeds involves significant specific features that are detailed below.
Upon contact between the liquid feed and the metal oxides, part of the feed is vaporized but coke settles on the particles due to the thermal cracking resulting from the liquid fuel being exposed to very high temperatures. The heavier the feeds, the more they tend to form large amounts of coke. Thus, on a diesel fuel or a vacuum distillate, the amount of coke formed is of the order of 1 to 20% of the feed injected. On an atmospheric residue or a vacuum residue, the amount of coke formed ranges from 10 to 80% depending on the nature of the feed injected. This coke formation depends on the nature of the feeds (coke precursor concentration, which can be determined by measuring the asphaltene content or the Conradson carbon content). It also depends on the contacting conditions (temperature, ratio of hydrocarbon flow rate to oxide flow rate, droplet diameter, particle diameter, etc.). After contact between the feed and the metal oxides, two types of combustion reaction between the hydrocarbons and the metal oxides take place, the first one, easier, resulting from a contact between the gaseous hydrocarbon and the oxide particles, and the second one, slower, resulting from the gasification of the coke that has settled on the particles to synthesis gas that will then burn rapidly with the metal oxides.
It is therefore important to minimize the formation of coke upon injection of the liquid feed in order to have combustion reactions as fast as possible.
On the other hand, considering the inevitable coke formation, it is important to make sure that the particles that return to the combustion zone stay there long enough to remove all the coke formed. Unlike the case where the feed is solid, it is indeed here impossible to consider separating the still coked oxide particles from the non-coked particles. In a solid feed combustion, the density difference, the size difference, or even the magnetic properties difference between the coal particles and the oxide particles is great. In the case of a liquid feed, the particles in the combustion zone are exclusively coked metal oxide particles and non-coked metal oxide particles, whose properties are very close.
The present invention allows to overcome all the aforementioned problems linked with the implementation of chemical looping combustion of liquid feeds.